--- *** UNDER CONSTRUCTION *** --- THE GEOID09 README FILE ----------------------- Version: January 16, 2009 drr The GEOID09 GEOID MODELS ------------------------ You have received these models on CD-ROM, or downloaded them from the National Geodetic Survey (NGS) web site or the NGS FTP site. Files you may have received include: INTG.EXE (PC) or The geoid interpolation program (source code is INTG (Sun) INTG.FOR) XNTG.EXE (PC) or Program for extracting, translating (ascii/binary) XNTG (Sun) and yielding statistics of geoid files (source code is XNTG.FOR) The following file names are valid for binary data (if, however, you downloaded the ASCII versions of these files, the suffix will be ".asc" rather than ".bin"): g2009u01.bin GEOID09 grid #1 for CONUS (40-58N, 230-249E) g2009u02.bin GEOID09 grid #2 for CONUS (40-58N, 247-266E) g2009u03.bin GEOID09 grid #3 for CONUS (40-58N, 264-283E) g2009u04.bin GEOID09 grid #4 for CONUS (40-58N, 281-300E) g2009u05.bin GEOID09 grid #5 for CONUS (24-42N, 230-249E) g2009u06.bin GEOID09 grid #6 for CONUS (24-42N, 247-266E) g2009u07.bin GEOID09 grid #7 for CONUS (24-42N, 264-283E) g2009u08.bin GEOID09 grid #8 for CONUS (24-42N, 281-300E) *** Alaska, Hawaii, Guam, CNMI, and American Samoa will be released soon *** *** Puerto/Rico & the U.S. Virgin Islands will be delayed until summer *** To Install: 1) Make a subdirectory on your hard disk. 2) Copy the various geoid files into that subdirectory. You need not put the geoid files in the same directory as the programs. (If you have also received GEOID09 model files, you may safely place them in the same directory as USGG2009, if you like.) To Execute (PC or Sun) Type INTG, and follow the prompts. To Terminate You can stop the program at any time using C. BUT, PLEASE DON'T START YET. PLEASE KEEP READING THIS DOCUMENT. File Structure --------------- The files (ASCII and binary) all follow the same structure of a one line header followed by the data in row-major format. The one line header contains 4 double words (real*8) followed by three long words (int*4). These parameters define the geographic extent of the area: SLAT: southernmost latitude in whole degrees minus sign (-) for South latitudes WLON: westernmost longitude in whole degrees EAST DLAT: distance interval in latitude in whole degrees (point spacing in E-W direction) DLON: distance interval in longitude in whole degrees (point spacing in N-S direction) NLAT: number of rows (starts with SLAT and moves northward DLAT to next row) NLON: number of columns (starts with WLON and moves eastward DLON to next column) IKIND: always equal to one (indicates data are real*4) After this one line header, the data follow. The first row represents the southernmost row of data with the first data point being in the SW corner. The row is NLON values wide spaced at DLAT intervals, and then increments to the next row which is DLAT to the north. This continues until the last row where the last value represents the NE corner. The easternmost longitude is = WLON + (NLON - 1) * DLON, while the northernmost latitude is = SLAT + (NLAT - 1) * DLAT. Check The Byte Counts of all Downloaded Files --------------------------------------------- Before beginning, it will be useful to ensure that all files you have received are the correct size. (Download problems are often manifested by incorrect byte counts in the files). Check with the list below to make sure your files match these numbers exactly. PC or Sun Data: g2009u**.bin 4,933,728 bytes g2009a**.bin 5,540,208 bytes g2009h01.bin 607,968 bytes g2009g01.bin 304,848 bytes g2009s01.bin 521,328 bytes ASCII Data: g2009u**.asc 12,488,896 bytes (uncompressed) g2009a**.bin 14,024,273 bytes g2009h01.bin 1,539,044 bytes g2009g01.bin 771,794 bytes g2009s01.bin 1,319,917 bytes FORTRAN Source Code: INTG.FOR 88,980 bytes XNTG.FOR 31,158 bytes PC executables: INTG.EXE 299,008 bytes XNTG.EXE 282,624 bytes Sun executables: INTG 466,692 bytes XNTG 426,924 bytes How Program INTG Works ----------------------- The various geoid height grids are stored in the ".bin" files. Program INTG will prompt you for the name of the directory where you have chosen to store the .bin files, as well as prompting you for which geoid model you wish to use. You can operate with as few as one .bin file, or as many as 14. When the program interpolates a given point, it checks an internal list of .bin boundaries, and uses the earliest list entry whose boundaries contain that point. The order in which the .bin file names appear on the opening screen indicates the order in which the .bin files are searched. When running program INTG.EXE (PC) or INTG (Sun), the latitude and longitude of each point must be input. The GEOID03 models are heights above the NAD 83 ellipsoid. However, latitudes and longitudes in the ITRF00/GRS-80 and WGS84 systems are very close to those of the NAD 83 system (with only 1-2 meters of horizontal shift.) So any of these types of latitude and longitude (NAD 83, ITRF00, WGS84) may be input, without affecting the interpolated geoid value. This does *not* imply that the geoid heights are heights above a different ellipsoid. Using NAD 83 latitudes and longitudes interchangeably with ITRF00/GRS-80 or WGS84 latitudes and longitudes is merely an acceptable horizontal approximation. GEOID09 geoid heights will always be above the NAD83 ellipsoid. Do *NOT* use NAD 27 latitudes and longitudes. The horizontal shifts between NAD 83 and NAD 27 can exceed 100 meters, causing a noticeable difference in the interpolated geoid value. To convert from NAD 27 to NAD 83 latitudes and longitudes you may use program NADCON, available from NGS, or CORPSCON, available from the Army Corp of Engineers: - either http://crunch.tec.army.mil/#products select CORPSCON, - or http://crunch.tec.army.mil/software/corpscon/corpscon.html Data Input ---------- You can key data by hand, point by point, or you can create an input file using a text editor. Several file formats are provided, including the NGS "Blue Book" format. These formats are detailed in a "Help" menu option which appears if you specify that you wish to use an input file. Data Output ----------- Results may be collected into an output file. There is no default output file name. The format of the output file is linked to the format of the input file to maintain consistency. If, however, you input your data by keyboard, and ask for an output file, the format of that output file will be in the format known as "Free Format, Type 1". The GEOID09 Model ----------------- The GEOID09 model is known as a hybrid geoid model, combining gravimetric information with GPS ellipsoid heights on leveled bench marks. The GEOID09 model was developed to support direct conversion between NAD 83 GPS ellipsoidal heights and NAVD 88 orthometric heights. When comparing the GEOID03 model with GPS ellipsoidal heights in the NAD 83 reference frame and leveling in the NAVD 88 datum, it is seen that GEOID09 has roughly a 1.5 cm absolute accuracy (one sigma) in the regions of GPS on Bench Mark coverage. In those states with sparse (150km+) GPS on Bench Mark coverage, less point accuracy may be evident; but relative accuracy at about a 1 to 2 part-per-million level, or better, should still be obtained. For users with less stringent accuracy requirements, simple height conversions with GEOID09 can be sufficient. For users with more stringent accuracy requirements, please see the section entitled "Deriving Orthometric Heights From GPS", later in this document. Users should be aware that GPS ellipsoid height error, by itself, can be significantly greater than error in geoid height differences. GPS on Bench Mark Coverage ----------------------------------------- As of the date of computation of GEOID09, all 48 of the Conterminous United States had been incorporated into the National Readjustment of 2007, Which realized the new reference framework employed at NGS: NSRS 2007. These points represent a tightly connected set of ellipsoid heights with Error estimates. Some points came in after the NRA 2007 was being processed and these were adjusted into older reference frames. Data that cam in after the NRA 2007 posted have been adjusted into the NSRS 2007 but do not have error estimates assigned. Both these issues are being addressed and further error assessment is being made to incorporate more data. See the link to the NGS webpage for GPS on BM’s in the Beta-GEOID09 model for a discussion of these issues The resulting model has a good spatial distribution and better reflects local geoid signal where the GPSBM data are close enough for modeling. GEOID09 in Alaska, Hawaii, Guam/CNMI, American Samoa, and PR/VI ----------------------------------------------------------------      Due to poorer data coverage, error estimates for GEOID09 in this region is larger.  Long-wavelength errors may be as large as 4-5 parts-per-million in some areas.  Particular care must be used in computing heights in the tectonically active areas in southern Alaska.  Crustal motion may exceed 1 meter even after accounting for the shift of the 1964 Prince William Sound Earthquake. Sparse data are available and, in conjunction with data available in Canada, A hybrid model was first realized in GEOID06. This is followed up here using the USGG2009 model for Alaska as a reference. In Hawaii, no vertical datum exists as of yet. The USGG2009 model was adjusted to account for NAD 83 as a reference ellipsoid but the vertical datum is essentially the gravimetric geoid. Guam and the islands in the Commonwealth of the Northern Marianas Islands each have their own vertical datum and the common model fades between them in offshore regions. American Samoa also has a vertical datum and the model there reflects the separation between NAD 83 and that local datum. Puerto Rico and the U.S. Virgin Islands represent a special case. These models Will be released later to allow for aerogravity and surface gravity observations To be incorporated into a very refined gravimetric geoid. The intent is to develop such a model to serve as the Vertical Datum (in lieu of a tide gage and a network of points). Puerto Rico does have its own datum, so some effort will be made to accommodate that, but the gravimetric geoid over the USVI is intended to become the first official gravimetric geoid based vertical datum. Deriving Orthometric Heights From GPS ------------------------------------- One key problem is deciding which orthometric height datum to use. NGVD 29 is not a sea-level datum, and the heights are not true orthometric heights. The datum of NAVD 88 is selected to maintain reasonable conformance with existing height datums, and its Helmert heights are good approximations of true orthometric heights. And, while differential ellipsoidal heights obtained from GPS are precise, they are often expressed in the NAD 83 datum, which is not exactly geocentric. In addition, GEOID03 rests upon an underlying EGM96 global geopotential model, and EGM96 does possess some error of commission. This leads to a warning: Do not expect the difference of a GPS ellipsoidal height at a point and the associated GEOID03 height to exactly match the vertical datum you need. The results will be close when converting NAD 83 GPS ellipsoidal heights into NAVD 88 elevations; but, maybe not accurate enough for your requirement. However, one can combine the precision of differential carrier phase GPS with the precision of GEOID03 height differences, to approach that of leveling. Include at least one existing bench mark in your GPS survey (preferably many bench marks). The difference between the published elevation(s) and the height obtained from differencing your adopted GPS ellipsoidal height and the GEOID03 model, could be considered a "local orthometric height datum correction." If you are surveying an extensive area (100+ km), and you occupy a lot of bench marks, then you might detect a trend in the corrections up to a one part-per-million level. This may be error in the GEOID03 model. We do not currently consider geoid-corrected GPS orthometric heights as a substitute for geodetic leveling in meeting the Federal Geodetic Control Subcommittee(FGCS) standards for vertical control networks. Studies are underway, and many less stringent requirements can be satisfied by geoid modeling. Widespread success has been achieved with the preceeding models, GEOID99, GEOID96, GEOID93 and GEOID90. The XNTG Utility Program ------------------------ The XNTG program can perform various functions, none of which are required to use the INTG program. The functions of XNTG are the extraction of sub-grids from the provided geoid grids, the translation between ASCII and binary grids, and the reporting of basic statistics for geoid grids. Future Plans ------------ New gravimetric and hybrid geoid models will be generated in the next year (USGG2006 and GEOID06) for all U.S. regions including the conterminous United States. These models may adopt newer global gravity field models derived from the GRACE mission as well as improved terrain and gravity field information. For More Information -------------------- For Products Available From the National Geodetic Survey: Information Services Branch National Geodetic Survey, NOAA, N/NGS12 301-713-3242 fax: 301-713-4172 For Information on GEOID03 and Future Research: Dr. Daniel Roman National Geodetic Survey, NOAA, N/NGS6 301-713-3202 email: dan.roman@noaa.gov Dr. Yan Ming Wang National Geodetic Survey, NOAA, N/NGS6 301-713-3202 email: yan.wang@noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/GEOID03/index.html end g2009rme.txt